The present invention relates to aqueous dispersions which are essentially free from protective colloids or emulsifiers, comprising in dispersed form
A. a polyurethane (A) comprising hydrophilic groups, which bring about the water-dispersibility of the polyurethane, this polyurethane (A) being essentially free from Cxe2x80x94C double bonds, and
B. a prepolymer having a content of from 0.1 to 1 mol of free-radically polymerizable Cxe2x80x94C double bonds per 100 g of prepolymer.
The invention also relates to processes for their preparation and to their use for coating articles.
Aqueous dispersions which can be used to give articles a surface which is resistant to mechanical stress and which withstands attacks by solvent, by coating these articles with the dispersion and then curing the coating by irradiation with light, are widely known.
For this purpose, DE-A-40 31 732 recommends aqueous dispersion mixtures where one component of the mixture is a dispersion of a polyurethane having hydrophilic groups which allow the polyurethane to be dispersed even in the absence of dispersing auxiliaries. The other dispersion comprises a dispersed prepolymer having free-radically polymerizable Cxe2x80x94C double bonds, this prepolymer being self-dispersible or being dispersed with the aid of emulsifiers or protective colloids.
DE-A-42 28 713 discloses radiation-curable binders. To prepare them it is recommended to disperse, in water, a mixture of a polyurethane which carries anionic groups and, as free-radically polymerizable double bonds, (meth)acryloyl groups, and a radiation-curable binder which contains (meth)acrylate groups. In this case the polyurethane serves as emulsifier.
DE-A-39 00 257 describes mixtures comprising polyurethanes modified with hydrophilic groups and with free-radically polymerizable double bonds, and free-radically curable synthetic resins (e.g. (meth)acryloyl-containing polyurethanes), and their dispersion in water.
These known, aqueous, radiation-curable dispersions, however, have inadequate stability on storage, or the irradiated or nonirradiated coatings prepared from these dispersions are still in need of improvement as far as properties such as hardness, chemical resistance, flexibility, adhesion to the substrate, sandability and surface quality are concerned, especially when they are extended with other polymer dispersions, for example styrene-acrylate or styrene-butadiene dispersions.
The properties of the nonirradiated coatings are likewise of importance, since on articles of complex geometry not every area is reached to a sufficient extent by the radiation which brings about the curing of the coating. Although the mechanical properties of the nonirradiated coatings cannot match the high level of the irradiated coatings, the former should at least be dust-dry and firm to the touch. In this respect too the known dispersions are still in need of improvement.
It is an object of the present invention, therefore, to remedy the deficiencies described above.
We have found that this object is achieved by the aqueous dispersions defined at the outset.
The polyurethane component (polyurethane A) of the molecular mixture is preferably composed of
a) diisocyanates having 4 to 30 carbons,
b) diols of which
b1) from 10 to 100 mol-%, based on the overall amount of the diols (b), have a molecular weight of from 500 to 5000 and
b2) from 0 to 90 mol-%, based on the overall amount of the diols (b), have a molecular weight of from 60 to 500 g/mol,
c) monomers, other than the monomers (a) and (b), which have at least one isocyanate group or at least one group which is reactive toward isocyanate groups, and which additionally carry at least one hydrophilic group or a potentially hydrophilic group, so as to bring about the water-dispersibility of the polyurethanes,
d) if desired, further polyfunctional compounds, other than the monomers (a) to (c), which have reactive groups which are alcoholic hydroxyls, primary or secondary aminos or isocyanate groups, and
e) if desired, monofunctional compounds, other than the monomers (a) to (d), which have a reactive group which is an alcoholic hydroxyl, a primary or secondary amino or an isocyanate group.
Suitable monomers (a) are the aliphatic or aromatic diisocyanates customarily employed in polyurethane chemistry. Preference is given to the monomers (a) or mixtures thereof, which are also mentioned as monomers (a) in DE-A-195 21 500.
Suitable monomers (b) and (d) are preferably those specified as monomers (b) and (d) in DE-A-195 21 500.
In order to make the polyurethanes dispersible in water, they are composed in addition to components (a), (b) and (d) of monomers (c), which are different from the components (a), (b) and (d) and carry at least one isocyanate group or at least one group which is reactive toward isocyanate groups and, furthermore, a hydrophilic group or a group which can be converted into hydrophilic groups. In the following text the term xe2x80x9chydrophilic groups or potentially hydrophilic groupsxe2x80x9d is abbreviated to xe2x80x9c(potentially) hydrophilic groupsxe2x80x9d. The (potentially) hydrophilic groups react with isocyanates substantially more slowly than do the functional groups of the monomers used to construct the polymer main chain.
Preferred monomers (c) are likewise those referred to as monomers (c) in DE-A-195 21 500.
The proportion of components having (potentially) hydrophilic groups among the overall amount of components (a), (b), (c), (d) and (e) is generally such that the molar amount of (potentially) hydrophilic groups, based on the amount by weight of all monomers (a) to (e), is from 80 to 1200, preferably from 140 to 1000 and, with particular preference, from 200 to 800 mmol/kg.
The (potentially) hydrophilic groups can be nonionic or, preferably, (potentially) ionic hydrophilic groups. It is preferred to operate without effective amounts of nonionic groups.
The content of nonionic hydrophilic groups, if included, is in general up to 5, preferably up to 3 and, with particular preference, up to 1% by weight, based on the amount by weight of all monomers (a) to (e).
Monomers (e), which can be used if desired, are monoisocyanates, monoalcohols and mono-primary and -secondary amines. In general their proportion is not more than 10 mol-%, based on the overall molar amount of monomers. These monofunctional compounds usually carry further functional groups, such as carbonyls, and are used to introduce functional groups into the polyurethane which allow dispersion or crosslinking or other polymer-analogous reaction of the polyurethane.
Monomers containing at least one isocyanate group or at least one group which is able to react with isocyanate groups in an addition or condensation reaction, and also Cxe2x80x94C double bonds, are employed not at all or only in insignificant amounts, i.e. only in amounts such that the properties of the end product are not thereby influenced.
In the field of polyurethane chemistry it is widely known how to adjust the molecular weight of the polyurethanes by choosing the proportions of the co-reactive monomers and the arithmetic mean of the number of reactive functional groups per molecule.
Normally, components (a) to (e) and their respective molar amounts are chosen so that the ratio A:B, where
A) is the molar amount of isocyanate groups and
B) is the sum of the molar amount of the hydroxyls and the molar amount of the functional groups which are able to react in an addition reaction with isocyanates,
is from 0.5:1 to 2:1, preferably from 0.8:1 to 1.5 and, with particular preference, from 0.9:1 to 1.2:1. With very particular preference the ratio of A to B is as close as possible to 1:1.
Furthermore, the proportion of monomers (a) is preferably chosen so that the proportion of the monomers (a) among the monomers (a) to (e) is from 20 to 70% by weight.
The monomers (a) to (e) which are employed carry, as an arithmetic mean, usually from 1.5 to 2.5, preferably from 1.9 to 2.1 and, with particular preference, 2.0 isocyanate groups and/or functional groups which are able to react with isocyanates in an addition reaction.
The polyaddition of components (a) to (e) takes place in general at from 20 to 180xc2x0 C., preferably from 50 to 150xc2x0 C., under atmospheric pressure or under the autogenous pressure.
The reaction times required may extend from a few minutes to a number of hours. In the field of polyurethane chemistry it is known how to influence the reaction by a large number of parameters, such as temperature, monomer concentration, monomer reactivity.
To accelerate the reaction of the isocyanates it is possible to use the customary catalysts, such as dibutyltin dilaurate, tin(II) octoate or diazabicyclo[2.2.2]octane.
Suitable polymerization apparatus comprises stirred vessels or the otherwise customary polymerization apparatus.
Preferred solvents are of infinite miscibility with water, have a boiling point of 40 to 100xc2x0 C. under atmospheric pressure, and react slowly if at all with the monomers.
The dispersions are in most cases prepared by one of the following processes:
In the acetone process, an ionic polyurethane is prepared from components (a) to (c) in a water-miscible solvent which boils at below 100xc2x0 C. under atmospheric pressure. Water is added until a dispersion is formed in which water is the coherent phase.
The prepolymer mixing process differs from the acetone process in that rather than a fully reacted (potentially) ionic polyurethane it is a prepolymer containing isocyanate groups which is prepared first of all. In this case the components are chosen such that the above-defined ratio A:B is greater than 1.0 to 3, preferably from 1.05 to 1.5. The prepolymer is first dispersed in water and then crosslinked, possibly by reacting the isocyanate with amines which carry more than 2 amino groups which are reactive toward isocyanates, or is chain-extended with amines which carry 2 amino groups which are reactive toward isocyanates. Chain extension also takes place when no amine is added. In this case, isocyanate groups are hydrolyzed to amine groups, which react with residual isocyanate groups of the prepolymers and so extend the chain.
If a solvent has been used in preparing the polyurethane, it is usual to remove the majority of the solvent from the dispersion, for example by distillation under reduced pressure. The dispersions preferably have a solvent content of less than 10% by weight and are, with particular preference, free from solvents.
Also dispensed with is the use of customary dispersing auxiliaries, i.e. emulsifiers or protective colloids, in effective amounts. If such dispersing auxiliaries are used at all, the amounts are less than 10, preferably less than 5, and, with particular preference, less than 3% by weight, based on the prepolymers (B).
Examples of suitable prepolymers (B) are those referred to in DE-A-40 31 732, and therein as component (B), or in DE-A-196 02 071, but with the exception of those which are self-emulsifiable. This means that they are essentially free from hydrophilic groups, such as ionic groups, or nonionic hydrophilic groups such as polyethylene oxide units.
In the case of the prepolymers (B) the polymer main chain is formed by ester, ether or urethane repeating units. They contain in general from 0.1 to 1, preferably from 0.1 to 0.5 mol of free-radically polymerizable double bonds per 100 g of prepolymer (B), with particular preference being given to methacryloyl or acryloyl groups.
The prepolymers (B) possess in general a number-average molecular weight of from 300 to 10,000, preferably from 300 to 3000 and, with particular preference, from 300 to 1500. The molecular weights can be determined, for example, by gel permeation chromatography (GPC) against a polystyrene standard.
The prepolymers (B) are employed without solvent or with a solvent content of less than 30% by weight.
The weight ratio of polyurethane (A) to prepolymer (B) is judiciously from 0.3:1 to 99:1, preferably from 1:1 to 99:1 and, with particular preference, from 2:1 to 20:1.
The prepolymer is preferably added to the polyurethane (A) or to its prepolymer before dispersing. However, it is also possible first to prepare an aqueous dispersion of a polyurethane (A) and then to mix this dispersion, at any desired point in time prior to the use of the dispersion, with the prepolymer (B), for example by stirring the prepolymer (B) into the aqueous dispersion.
It is assumed that the polyurethane (A) and the prepolymer (B) are present in the dispersion in the form of a molecular mixture and not in the form of a dispersion containing in one discontinuous phase only the polyurethane (A) and in a further discontinuous phase the prepolymer (B).
The dispersions generally have a solids content of from 20 to 70, preferably from 25 to 60 and, with particular preference, from 35 to 45% by weight. The viscosity is usually from 10 to 1000 mPas, measured at 23xc2x0 C. and at a shear rate of 250 sxe2x88x921.
The dispersions generally contain less than 10% by weight of solvent and with preference are virtually solvent-free.
The novel dispersions can comprise customary commercial auxiliaries and additives, such as wetting agents, antifoams, matting agents, emulsifiers, photoinitiators, thickeners and thixotropic agents, and colorants such as dyes and pigments.
Furthermore, these dispersions can be blended with other aqueous polymer dispersions, the solids content of such dispersions relative to the solids content of the novel dispersions being from 1:10 to 10:1.
These blending dispersions are, for example, generally known dispersions of copolymers of olefins, methacrylates or acrylates, such as styrene-butadiene or styrene-acrylate copolymers, or a customary polyurethane dispersion.